1. Field of Invention
This invention relates to radio frequency antennas, specifically to such antennas which are capable of operating over a wide frequency range.
2. Description of Prior Art
An antenna is usually positioned as high as practical and connected to a transmitter or receiver by way of a feed line. Antennas for use at radio frequencies are effective over a limited frequency range. The optimum operating frequency of an antenna is determined by its length. The lower the operating frequency, the longer the antenna must be.
When operation is required over a wide frequency range it is common to use multiple antennas with each antenna optimized for a specific narrow band of frequencies. The desired antenna is manually selected by a switch or other means that connects that antenna's feed line to the transmitter or receiver.
Since horizontal dipole antennas have a preferred direction of operation, more than one antenna of the same frequency band may be erected in order to achieve coverage in all directions. This creates the need for more feed lines, and more antenna selection switches.
A simple dipole antenna is often made of wire. The dipole may be connected to a transceiver by way of a coaxial cable feed line. The cost of the coaxial feed line is the most expensive part of such an antenna. When many separate antennas are needed in order to cover a broad range of frequencies or different directions, the cost of the feed lines can become significant.
When multiple antennas are used in close proximity, they can interfere with each other. The interference can be a disruption of the normal impedance of the antenna. The interference can also be a disruption of the normal radiation pattern of the antenna.
Another problem with multiple antennas is that a large physical space is required to accommodate them. Still another problem is the number of supports required to hold the multiple antennas as high as practical.
Because of the above mentioned problems, other methods have been devised to use a single antenna and feed line over a wide frequency range. One such method is to use an electrical network to match the impedance of an antenna of the incorrect length to the output impedance of the transceiver. This network is sometimes incorrectly called an antenna tuner. There are several problems with the antenna matching technique:                a) Some transmitter power is lost in the matching network and is not radiated by the antenna.        b) Considerable transmitter power can be lost in the feed line.        c) Undesirable radiation patterns with multiple lobes and deep nulls occur at frequencies above the resonant frequency of the antenna.        d) A slight change in operating frequency requires readjustment of the matching network.        e) Readjustment of the matching network takes time.        f) Antenna matching networks can be expensive, physically large, and cumbersome to operate.A description of antenna matching techniques can be found in “The ARRL Antenna Book” 16th edition, pages 25-1 to 25-14.        
Another method for using a single antenna and feed line over a wide frequency range is the trap antenna. This type of antenna employs networks of inductors and capacitors placed at key points along the length of the antenna. The networks are commonly called traps. One pair of traps is required for each band of frequencies on which the antenna is to operate. There are several problems with the trap antenna:                a) The large size and weight of the traps causes considerable wind load and support problems.        b) The traps have losses which prevents some of the transmitter power from being radiated by the antenna.        c) The traps are expensive to construct.        d) The bands of operation are narrow compared to a normal dipole.        e) The individual traps require tuning.        f) There is interaction between the traps which makes it difficult to get the antenna adjusted to all the desired operating points.A description of trap antennas can be found in “The ARRL Antenna Book” 16th edition, pages 7-8 to 7-12.        
Another method of making a single antenna and feed line operate over a wide frequency range is to place an antenna tuning network at the antenna end of the feed line as disclosed in U.S. Pat. Nos. 4,201,990 and 4,564,843. The purpose of this type of technique is to match the impedance of the non-resonant antenna to the impedance of the feed line. There are several problems with this technique:                a) The size and weight of the tuning network causes wind loading and support problems.        b) Other wires beside the feed line must run to the tuning network to power it and to control it.        c) Undesirable radiation patterns with multiple lobes and deep nulls occur at frequencies above the resonant frequency of the antenna.        d) The network must often be readjusted when even small changes in frequency are made.A variation of the above method is disclosed in U.S. Pat. No. 4,924,238. In this method the elements of the tuning network are distributed along the length of a helically wound antenna structure. This method has all of the problems described above.        